An easily accessible Re-based catalyst for the selective conversion of methanol: evidence for an unprecedented active site structure through combined operando techniques

Abstract

Heterogeneous Re/SiO(2) catalysts prepared using a one pot sol-gel synthesis were found to display high activity in the direct, selective methanol conversion to methylal, which is correlated to an unprecedented rhenium oxide structure.

Surface ReOx Sites on Al2O3 and Their Molecular Structure–Reactivity Relationships for Olefin Metathesis

“…The absence of strong peaks at high R distance is consistent with the isolated nature of the (O) 3 ReO-Si-(phenyl) 3 reference compound. With the decrease in the number of ReO bonds (3 in trioxo (O) 3 ReO-Si-(phenyl) 3 , 2 in iododioxobis (triphenylphosphine) rhenium, and 1 in trichlorooxobis (triphenylphosphine) rhenium), the intensity of the EXAFS peak at ∼1.2 Å decreases. The crystalline bulk ReO 3 reference compound contains its Re−O peak at ∼1.5 Å reflecting the longer metal−oxygen bond length in ReO 3 and absence of ReO bonds in this structure.…”

“…12,13 TOF-SIMS analysis of calcined supported ReO x / Al 2 O 3 catalysts also demonstrated that the surface ReO x species are essentially isolated on the alumina support. 29 The intense Re L 1 XANES pre-edge feature of the supported ReO x /Al 2 O 3 catalysts approaches that of the (O) 3 ReO-Si-(phenyl) 3 reference compound in (see Figure 2), which strongly suggests that the surface ReO x sites possess ReO 4 coordinated with only slightly different symmetry than C 3v from the reference compound. The presence of the two long Re···O bonds (2.35 Å) bonds for the deformed (110_1) surface ReO 4 structure does not appear to influence its first coordination shell and, thus, its XANES pre-edge features.…”

“…12,13 Bare et al concluded that isolated trioxo (O) 3 Re−O−Al species are present for calcined supported rhenia/alumina catalysts 12 mainly from EXAFS analysis. In contrast, Scott et al postulated isolated penta-coordinated dioxo (O) 2 Re(−O-Al) 3 from XANES/ EXAFS measurements and DFT calculations employing cluster models. 13 The precise coordination of the isolated surface Re 7+ oxide species on alumina is of great importance to understand the remarkable low-temperature olefin metathesis activity of this catalyst system.…”

“…The (O) 3 ReO-Si-(phenyl) 3 reference compound with isolated rhenia sites exhibits a strong peak at ∼1.2 Å from the terminal In situ Re L 1 XANES spectra for the dehydrated supported 3% (blue) and 15.6% (olive) ReO x /Al 2 O 3 catalysts. The XANES spectra of the reference compounds, trioxo(triphenylsilyloxy) rhenium(+7) (red), iododioxobis (triphenylphosphine) rhenium(+5) (magenta), trichlorooxobis (triphenylphosphine) rhenium(+5) (orange), and ReO 3 rhenium(+6) oxide (black) are taken under ambient conditions.…”

“…Strong deviations from the MO 6 O h symmetry result in a sharp preedge feature in the Re L 1 edge XANES reflecting the absence of inversion symmetry. 17 The trioxo(triphenylsilyloxy) rhenium (+7) reference compound consists of ReO 4 coordinated isolated (O) 3 ReO-Si-(phenyl) 3 units containing C 3v symmetry ( Figure S4) and exhibits a sharp Re L 1 XANES pre-edge because of the absence of inversion symmetry. The other rhenia reference compounds do not exhibit a strong Re L 1 XANES pre-edge.…”